Field of the Disclosure
This disclosure is directed to hybrid sulfur particles useful as cathode active material in a metal ion battery. This disclosure is also directed to a cathode containing the hybrid sulfur particles and an electrochemical cell or battery containing the cathode.
Discussion of the Background
An ongoing objective in the commercial development of electric vehicles and portable electronics is to provide batteries with higher energy densities than currently available with state of the art lithium ion batteries. One approach in achievement of this objective is to couple a metal anode, such as lithium or magnesium, with a high capacity conversion cathode, such as sulfur or oxygen, without sacrificing cycle life and rate capability. Sulfur is highly attractive because it is economical, highly abundant and offers a charge capacity that is an order of magnitude higher than conventional insertion lithium ion cathodes. However, sulfur is electrically insulating and exhibits unacceptably high mass loss during cycling due to the formation of polysulfide reduction intermediates which are highly soluble in an electrolyte and do not return to the cathode during a recharge cycle.
Thus, although elemental sulfur has been under investigation as a cathode active material in conjunction with metal anodes for more than 50 years, in order to obtain viable commercial sulfur cathode energy storage and supply source, these two fundamental challenges must be overcome. The first challenge is to enhance the conductivity of elemental sulfur. Unlike commercial lithium ion cathodes (LiCoO2) which possess a high electronic conductivity and do not require significant addition of conductive additives, sulfur is an effective insulator which is 1 billion times less conductive than LiCoO2. Therefore, any cathode active material based on elemental sulfur must be enhanced with conductive additives.
The second challenge is to control the diffusion of polysulfide intermediates formed during cycling. During discharge, sulfur reduces in a stepwise manner by forming a series of polysulfide intermediates which are ionic in nature and solvate easily in the electrolyte. This causes mass loss of active material upon cycling. Even today, while approaches mitigate these fundamental challenges of low conductivity and dissolution of polysulfides, they also diminish the superior charge capacity of sulfur.
As indicated, one problem deriving from the insulating nature of sulfur is the need for high loadings of conductive additives to improve the overall electronic conductivity. This results in low sulfur content in the cathode and thus reduced energy capacity. A second problem is the slow rate of operation due to the low electronic conductivity of sulfur and the low ionic conductivity of the reduced product, Li2S. Third, the diffusion of ionic polysulfides limits cycle life due to anode passivation and mass loss from the cathode.
Extensive research efforts have been devoted to developing methods to enhance the conductivity of elemental sulfur and to control the diffusion of polysulfide intermediates formed during cycling. Researchers have studied conductive hosts infused with sulfur and polymer-coated sulfur composites. Since the pioneering findings by Nazar who demonstrated the benefit of infusing sulfur into ordered mesoporous carbon, various micro/nano carbon hosts including spheres, nanofibers, graphene oxide and carbon paper, have been investigated as conductive hosts to contain the sulfur active material (Nazar et al. Nature Materials, 2009, 8, 500-506). Manthiram has recently demonstrated a microporous carbon interlayer with pore sizes matching the dimensions of the polysulfide ions (Manthiram et al. Nature Communications, 2012, 3, 1166). Tarascon infused sulfur into metal organic frameworks (MOF) with hopes of benefitting from interactions between the polysulfides and the MOF oxide surface (Tarascon et al. Journal of the American Chemical Society, 2011, 133, 16154-16160). While these approaches improve the conductivity of the sulfur cathode, they are still plagued by diffusion of polysulfides out of the host pores which limits cycle life. In 2012, Amine obtained a SeS2 carbon nanotube composite starting from commercially available SeS2 delivering 512 mAh/g at 50 mA/g after 30 cycles (Amine et al. Journal of the American Chemical Society, 2012, 134, 4505-4508). Various composites of SeSy (y=2 or 7) carbon nanotube composites have been prepared and evaluated. The discharged capacities varied from 571 to 833 mAh/g at 50 mA/g after 50 cycles. In addition, Li et al. have explored the preparation of Se/S composites infused into porous carbon which delivers capacities of 910 mAh/g at 1 A/g over 500 cycles (Li et al. Energy and Environmental Science, 2015, 8, 3181-3186).
In U.S. application Ser. No. 14/489,597, filed Sep. 18, 2014, the present research group has described novel encapsulated sub-micron sulfur particles formed in the presence of a mixed hydrophilic/hydrophobic copolymer. The resulting encapsulated sulfur sub-micron core particle is coated with a membrane of layers of self-assembling conductive polymer layers, each successive layer having a charge opposite to the previous layer.
However, the major disadvantage of all these approaches is that they require a carbon matrix to enhance conductivity of the active material and thus dilutes the sulfur capacity of the cathode.
Thus, an object of the present disclosure is to provide a sulfur particle having a balance of high capacity and good conductivity which is suitable for utility as a cathode active material.
A second object of the disclosure is to provide a cathode containing the particle as an active material which is suitable for utility in a battery having high capacity and high cycle lifetime.
A third object of the disclosure is to provide a battery which has sufficient capacity and lifetime to be a viable energy source for a vehicle.